The main objective of the proposed research project is the study of the mechanisms of a group of metabolic reactions which involve the alkylation of the carbon atom at position 5 of pyrimidine nucleotides. The research plan is based on the working hypothesis that the mechanisms of these and some related reactions involve the addition-elimination of an enzymic sulfhydryl group across the 5,6 double-bond of the pyrimidine nucleotide substrates. The problem is approached by studying the effects of specifically designed inhibitors on the kinetics of a few representative enzymic reactions, as well as by studying the effects of selective modifications of the active site sulfhydryl groups on the catalytic properties of the enzymes. The study is extended to investigate the correlation between the effects of the inhibitors observed in the isolated enzyme systems and their effects on nucleic acid metabolism in intact, growing cells.